The present invention relates to a method for encoding and detecting a carrier signal in a video signal for signaling purposes, and more particularly to methods and apparatus for determining an optimum level and placement of a carrier signal to be modulated into an active portion of a video signal so as to deter nefarious third parties from stripping the carrier signal out of the video signal and increase the detectability of the carrier signal within the video signal without noticeably decreasing the clarity of a picture represented by video signal to a viewer.
Various methods exist in the art for transmitting a carrier (or subcarrier) signal along with video signals, wherein the carrier signal is used for a variety of signaling purposes. Several of these methods transmit the carrier signals, such as in the form of auxiliary data, in the video signals by replacing active portions of the video signal with auxiliary data, such that users who view the video signal on their display devices (e.g., televisions) will see the effect of the carrier signal in the form of an icon, dot or other visual image or disturbance in the picture. Other methods transmit carrier signals in non-viewable scan lines of the video signal, such as in the vertical blanking interval (VBI). However, these scan lines may already contain other carrier signals such as signals that represent cueing information, timing information or closed captioning information and are prone to being stripped by programming operators prior to broadcast.
Another method for transmitting a carrier signal in video signals is described in U.S. Pat. No. 4,807,031 to Broughton et al. (“Broughton”) entitled “Interactive Video Method and Apparatus”, which relates generally to in-band video broadcasting of commands and other encoded information to interactive devices and is incorporated by reference herein. The invention described therein relates generally to interactive educational and entertainment systems, and is described in one embodiment in the context of television program control of toys located where there is a television receiver, as within a residence.
To encode control data, Broughton discloses a novel method of luminance or chrominance modulation of a video signal that creates a composite video signal, wherein the video signal is modulated with control data. The novel modulation method alternately raises and lowers the luminance/chrominance of paired adjacent horizontal scan lines to create a video subcarrier that contains the control data.
In Broughton, the video signal is not being replaced with other data, nor is the data being added as a separate signal along with the video signal. Rather, the video signal itself is modulated to carry the control data. Therefore, the control data is a part of, or contained within, the video signal and yet is imperceptible to the human eye. The encoding method also includes preview and remove circuitry to ensure suitability or the presence of data encoding and removal of data encoding, respectively.
The control data is transmitted either by television broadcast means, or by pre-recorded video players that are connected to a video display. The control data is then received by the video display where at least one video field of the video display is modulated by control data. The control data is then detected with either opto-electronic or radio frequency (RF) detection means that discriminate between the program material and the control data to detect the control data. The detected control data is further reproduced so that the control data can be used with an interactive device.
Improvements on the method of modulation described in Broughton are described in U.S. Pat. No. 6,094,228 to Ciardullo et al. and U.S. Pat. No. 6,229,572 to Ciardullo et al. (referred to collectively herein as “Ciardullo”). Both Ciardullo patents describe improved methods of signal modulation wherein the auxiliary data is inserted within the visual portion of a video signal by changing the luminance of paired scan lines in opposite directions. Instead of raising and lowering the intensity on the whole line as in Broughton, Ciardullo uses pseudo noise sequences to raise and lower the intensity on portions of a first line, where the line paired to the first line is modulated with the inverse pseudo noise sequences. Ciardullo thereby allows larger amounts of auxiliary data to be modulated in the video signal by use of the pseudo noise sequences. Ciardullo, which is owned by the assignee of the present invention, is incorporated by reference herein.
Improvements in the method of modulating data in the active portion of the video signal are disclosed in U.S. Pat. No. 6,661,905 to Chupp et al. (hereinafter “Chupp”). Chupp discloses a method of superimposing data on the visible portion of a video signal comprising the steps of analyzing an image defined by a video signal and forms of pixels to identify a data carrying parameter associated with each pixel, developing a chip characteristic table having digital values that represent the amplitudes of respective chips to be superimposed on the video signal at corresponding positions, each chip having a varying characteristic determined by the parameter combining the video signals with the chips using the derived chip amplitudes into a composite signal, and transmitting the composite video signal. Chupp is also owned by the assignee of the present invention and is incorporated by reference herein.
At the time of the present invention, analog display devices (e.g., NTSC televisions) operate by use of a fine pitch electron beam that strikes phosphors coating on an internal face of the cathode ray tube (CRT). The phosphors emit light of an intensity which is a function of the intensity of the beam striking it. A period of 1/60 second is required for the electron beam to completely scan down the CRT face to display a field of the image. During the following 1/60 second, an interlaced field is scanned, and a complete frame of video is then visible on the analog display device. The phosphors coating on the face of the tube is chemically treated to retain its light emitting properties for a short duration. Thus, the first area of the scanned picture begins to fade just as the electron beam retraces (i.e., during the vertical retrace) to the top of the screen to refresh it. Since the electron beam covers 525 lines 30 times per second, a total of 15,750 lines per second are viewed each second.
Broughton's method of encoding a carrier signal in a video signal and its improvements were generally intended for use with an analog display device. Upon receiving the video signal from the signal source, such a display device splits the video signal into sequentially transmitted images referred to as frames, whereby each frame of an NTSC television image has 525 horizontal scan lines. The display device scans 262.5 of the horizontal lines left to right and top to bottom by skipping every other line, thus completing the scan of a first field, and then retracing to the top of the image and scanning the remaining 262.5 lines, for a second field. The fields are interlaced at the display device and construct one complete frame. When the video signal is broadcast at 525 lines per frame and 30 frames a second there are 60 fields per second and a line frequency rate (i.e., the speed at which lines are refreshed) of 15,750 Hz (i.e., approximately 16 kHz).
The use of Broughton and other methods of encoding carrier signals may not be sufficiently robust for embodiments where there is a possibility that the carrier signal will be detected, altered or removed by an unauthorized party. Under Broughton, the unauthorized party may detect the frequency at which carrier signal is present and use an electronic device to strip out the carrier signal while substantially preserving the video signal. The detection, removal or alternation of the carrier signal may provide the unauthorized party with additional benefits or access to which the party would not otherwise be entitled, such as when the carrier signal is used to restrict unauthorized reproduction of the video signal.
The possibility of unauthorized detection, removal or alternation of carrier signals may be reduced under the present invention by spreading the resulting encoding frequency over a spectrum. Generally, spread spectrum technology is used with wireless communications in which the frequency of a transmitted signal is deliberately varied. The signal is thus transmitted over a greater bandwidth than if the signal did not have its frequency varied. Thereby, the signal is less likely to be disrupted if there is a significant amount of interference at a particular frequency. In addition, the spreading of the spectrum from a single frequency to multiple frequencies dramatically increases the difficulty of an unauthorized party interfering with or intercepting the carrier signal.
Since there is a frequency generated by adding the carrier signal to the video scan lines in a regular pattern, it is desirable to vary the locations and levels by which the intensity of the video signal is altered so that the resulting frequencies from modulating the video signal will occur over a wide range. Accordingly, there is a need in the art to modulate a video signal with a carrier signal over a spread spectrum wherein the presence of the carrier signal is detectable without paired lines, such that it is difficult to remove the carrier signal from the video signal without rendering the video signal unwatchable and the resulting picture distorted.
Although Broughton and its improvements have been frequently used and well received since their inception, the relative invisibility of the carrier signal in the picture of the display device and the ease of detecting the carrier signal by a detector or from the display device by a hand-held device can be yet improved. A slight tendency to visibility of the carrier signal in the active portion of the video signal may occur when the voltage of the carrier signal is increased for the purpose of increasing the carrier signal's detectability, as television viewers might then slightly perceive the effects of the carrier signal on the visible picture, such as a slight tendency of visible lines or a slight deterioration in the picture quality. Since one of the advantages of using Broughton is its invisibility to the human eyesight, any tendency of viewing the effects of the carrier signal is undesirable.
The invisibility challenge is typically resolved by reducing the voltage (i.e., as resultant luminosity) added to or removed from the selected video scan lines. However, lowering the overall signal intensity decreases the reliability of detecting the carrier signal. Despite the success of the technology of Broughton and its improvements, wherein the modulation of video with carrier signals results in at no worse than subliminal visual changes which are substantially invisible, there is a need in the art for a new method and system for modulating a video signal with a carrier signal wherein the signal is even more completely invisible and yet more reliably detected.
Modulated video signals are subject to tampering as the signal may be resized or otherwise altered such that the video signal is de-interlaced. When the video signal is re-interlaced, it may become altered such that the line to line differences between a first and a second field may be read erroneously by a detector such that the carrier signal is shifted so that it is no longer on the desired lines or is on undesirable lines. The detection, removal or alternation of carrier signals may provide the unauthorized party with additional benefits or access to which the party would not otherwise be entitled. Accordingly, there is a need in the art to modulate a video signal with a carrier signal wherein the presence of the carrier signal is also detectable by detecting the magnitude of line to line differences in a field of a video signal.